This invention relates to printing systems using a printing element on which the image is defined in terms of contiguous hydrophilic and relatively hydrophobic regions, and which is capable of serving as a printing plate or other analogous source of a transferrable ink image. More specifically, this invention relates to a novel printing system comprising a non-photosensitive, reusable printing surface suitable for use in a lithographic-type or other printing system, on which an ink image may be formed, refreshed, or completely reconfigured electronically, without a separate development or plate making step, without removal of the printing element, and without substantial interruption of the printing process.
In modern printing systems using printing plates such as letter-press and intaglio or gravure systems, the image portions of the printing plate are defined in terms of raised or recessed area of the plate surface which are made to carry ink. In planographic systems such as lithography, however, the image portions of the printing plate, i.e. those portions of the printing plate surface intended to carry ink, are formed at substantially the same surface level as the rest of the plate. Rather than depend upon the relative elevation of portions of the plate surface to define the ink-bearing image, planographic systems depend upon certain areas of the plate having a greater relative affinity for water than is shown by the remaining areas of the plate.
In a typical lithographic printing system, the relative immiscibility of grease and water is used to define and maintain the image and non-image areas of the printing plate. In standard lithographic printing systems where greasy-type or oleo inks are used, the lithographic plate is made oleophilic (grease-loving) and hydrophobic (water-hating) in image areas (i.e., those areas which will receive and transfer ink to the paper sheet or other material to be printed), and hydrophilic (water-loving) in the non-image areas. These latter areas, which are in image-complementary configuration, are sometimes referred to as "lithographically blank" areas, because they normally carry or transfer no ink. So long as sufficient water is present in these lithographically blank areas, no oleo-type ink will adhere to the plate in these non-image areas. By this arrangement, these hydrophilic, image-complementary areas of the plate will retain preferentially an aqueous fountain or dampening fluid applied to the plate to the exclusion of the remaining portions of the plate, and will thereby allow the greasy ink applied thereafter to adhere only to the oleophilic areas of the plate intended to carry the ink image.
Various techniques have been developed for establishing the hydrophilic and hydrophobic areas of the printing plate. The most popular method of establishing such image-defining areas is with the aid of light sensitive materials which tend to undergo chemical reactions when exposed to actinic light. In a typical process, when using negative-imaged films, the so-called "negative" plate is covered with a layer of a light sensitive diazo or photopolymeric formulation. Strong light energy passing through the negative film and striking the plate causes the diazo or photopolymeric formulation in the exposed or imaged areas of the plate to undergo a chemical change, e.g., to polymerize, forming thereby a hardened, hydrophobic, ink receptive area. The non-polymerized formulation in the unexposed or image-complementary areas of the plate is removed by washing the plate surface with a solution in which only the unexposed, non-polymerized formulation is readily soluble. These unexposed, washed areas are then treated with gum, i.e., a gum formulation containing gum arabic, carboxymethyl cellulose gum, or the like. Often, the non-polymerized formulation is washed away and the gum added in a single step. If a long wearing plate is desired, a thin film of a gum-containing material may be rubbed onto or otherwise applied to the plate and the plate surface washed with water, thereby causing a water insoluble layer of gum to be adsorbed onto the unexposed or image-complementary areas of the plate surface, and forming a highly hydrophilic surface which will wet readily with water, and will thereafter reject ink.
If a positive rather than a negative type film is used, the so-called "positive" plate is first sensitized with a light sensitive coating which degrades when exposed to actinic light. Exposure of the plate, via the positive film, then results in degradation of the coating in what will be the image-complementary (i.e., non-ink-carrying) portions of the image. The coated plate is chemically washed to remove the degraded areas of the coating. The plate is then baked to harden the coating in the image (i.e., ink-carrying) areas, and coated with a gum-containing material such as gum arabic or the like, as is done with the "negative" plate discussed above.
Systems using light sensitive materials customarily require the preparation of a photographically-generated film negative or positive transparency, as well as the careful projection of the image carried by the transparency onto the light sensitive surface of the plate. In certain systems, e.g., in so-called photo-direct systems, a plate may be exposed directly by the original copy without the need for an intermediate film transparency. In either case, however, it is usually necessary that the resulting plate be developed and rinsed and a finishing solution usually must be applied.
Electrostatic systems for generating a lithographic plate may be based on use of either a hydrophilic or a hydrophobic toner material. If, for example, a hydrophobic toner material is used, a plate surface comprising a photoconductive material which is hydrophilic is given a uniform electrical charge prior to being exposed to light striking the plate in image-complementary configuration. The light causes neutralization of the electrical charge in the illuminated areas of the plate. To develop the plate, a toner carrying a charge opposite to that of the remaining charged areas of the plate is then applied and made to stick to the plate surface. After fusing, the toned areas become hydrophobic, while the untoned areas remain hydrophilic. Use of a hydrophilic toner material employs analogous process steps with an initially hydrophobic plate surface.
The lithographic-type plates produced by the various techniques discussed above, as used in printing presses and processes of conventional design, generally exhibit substantial deficiencies which are well known and commonly encountered in the printing industry. Representative of these deficiencies are the following:
(1) inability to generate a high quality lithographic-type printing plate without film preparation steps or without elaborate plate exposure and development procedures;
(2) inability to reconfigure completely the image being printed by the plate without substantial interruption of the printing process or substitution of a second plate carrying the desired reconfigured image;
(3) inability to refresh or renew the oleophilic and hydrophilic areas of the image carried by the printing plate without substantial interruption of the printing process;
(4) inability to correct minor deficiencies in the image being printed by the plate--for example, those deficiencies caused by incomplete or unintended removal of material from the plate surface, or by foreign matter residing on the plate surface--without substantial interruption of the printing process;
(5) inability to correct substantial registration errors in the plate without re-plating;
(6) inability to print a continuously repeating pattern on a web substrate using a rotary-type press without a gap or seam between plate image pattern repeats and without the use of additional plates or ink heads;
(7) inability to print a pattern wherein the repeat length is greater than, or wherein the repeat length will not integrally divide into, the plate length or circumference of the plate roll;
(8) inability to eliminate roll shock, i.e., the mechanical interaction between the respective gaps of the plate and blanket rolls in rotary offset printing methods, which limits press speeds;
(9) inability to proof conveniently a freshly generated plate under true production conditions, using production inks, papers, etc.;
(10) inability to store the equivalent of a large library of printing plates for short or periodic printing runs without substantial maintenance and inventory costs;
(11) inability to generate a lithographic-type printing plate, which requires no separate developing process, or print imagery using a lithographic-type printing process, directly from a source of electronically-generated images such as a digital computer.
Attempts to overcome these and other deficiencies of existing systems generally have met with only limited success. Disclosed herein is a printing system employing a reusable printing plate which overcomes all of the above-listed deficiencies, as well as others associated with almost all photolithographic techniques, such as halation (i.e., imperfect light exposure caused by the reflective nature of the printing plate supporting base).
A substantially planographic plate suitable for service in a lithographic-type printing system is described herein which is comprised of an intrinsically hydrophilic plate material which supports a thin hydrophobic layer thereon. Also described herein is a method for generating, imaging, and using such a plate to print electronically generated images in various printing processes. According to the teachings herein, a method for generating a plate for use in a lithographic-type printing system comprises coating uniformly an intrinsically hydrophilic support surface with a thin hydrophobic layer of a suitable material, then selectively removing the material in a pre-determined configuration by means of an electronically addressable imaging system utilizing an electric spark discharge, a beam of electromagnetic energy (e.g., a laser beam), a beam of ionized particles, or other means. Alternatively, the hydrophilic plate surface may be first coated with a thin layer of a hydrophilic protective material, for example, a gum-containing material, prior to the application and selective removal of the material forming the hydrophobic layer. As additionally taught herein, suitable material for forming a hydrophobic layer may be directly, selectively applied to the plate in the desired configuration. Whether selectively removed or selectively applied, the hydrophobic layer material may be said to be arranged over the plate surface in a desired image-related configuration. These as well as other developments, all of which involve a reusable, easily re-imageable ink image generation surface useful in various printing processes, are described herein. As used herein, ink image generation surface is intended to mean the surface on which the ink image corresponding to the desired printed image is initially formed. This surface generally will be the surface on which a pre-ink latent image, i.e., an image defined in terms of adjacent hydrophilic and hydrophobic areas, is also initially formed. The term "imaging" is intended to mean the generation of this latent image, prior to the application of ink.
Described herein is a surface suitable for use, for example, as a planographic printing plate in either rotary or non-rotary printing systems wherein an electronically embodied image may be impressed directly onto the plate, without requiring the use of photosensitive materials or coatings, or without elaborate developing steps. In addition, the disclosed surface is re-usable, in the sense that a lithographic plate, for example, when imaged and used for printing in accordance with the teachings of this invention, may be re-imaged with the same or with a totally different image without the need for replacing the plate. In fact, an image having a length greater than (or not an integral divisor of) the circumference of the plate roll, where such roll is used, may be printed by changing the image associated with one portion of the plate roll while another portion of the roll is transferring an ink image to an offset roll or directly to a substrate.
Throughout this discussion, the terms "printing plate" or "plate" shall be used to describe a substantially flat, planographic surface capable of recording an image defined in terms of hydrophobic and relatively hydrophilic areas; such a surface may be the ink transfer surface associated with either a planar or curved lithographic printing plate, and may even be, for example, the print roll surface itself and not a separate, detachable entity usually associated with the term "plate." The printing plate may take the form of a planar surface, a cylinder, an endless belt, or other form. It is foreseen that the printing element as described herein may also comprise the printed product, e.g., the plate need not serve as an ink transfer surface, but as the printed substrate itself. In addition, other, non-planographic surfaces may be employed as well.
A method and apparatus is herein disclosed which can completely eliminate the costs associated with generating a plate using conventional photolithographic techniques, as well as the costs involved in maintaining a conventional plate library for short-run or periodic printing jobs. The necessity of replacing a plate when a sharpened, or slightly modified, or totally reconfigured image is desired is completely eliminated. The costs and limitations associated with having gaps in the plate used in rotary-type presses which cause a printing gap or seam in matter printed on long webs, as well as the mechanical shock associated with such plate gaps and the speed limitations such plate gaps impose, can be completely eliminated by imaging the roll surface as herein described, rather than imaging a separately attached printing plate of conventional design. Additionally, a series of pre-production run proofs may be generated inexpensively, and with the advantage that the proofs may be printed on the same machine, using the same plate, paper, inks, and many of the same press adjustments as the final production run, thereby eliminating any doubt whatsoever as to the appearance of the final printed image. Whatever adjustments are necessary to develop a satisfactory proof, regardless of their magnitude, can be made to the plate without removing the plate from the press, or having to make ready and install an entirely new plate.
The teachings herein may be used in a wide variety of printing applications, particularly where, for example, minimal costs for plate preparation, set up, storage, or inventory are desired, or where no gap or seam between plate images on a continuous printed substrate is desired. Because of the lack of any plate gap or seam, and any corresponding mechanical shock originating therefrom, the teachings herein are also particularly suited to applications wherein high speed printing (e.g., high speed rotogravure speeds) is desired.
Other features and advantages will become apparent from the following detailed description in which reference is made to the figures summarized below.